Gas Assist Spillovers & Overflows

Better Described As Gas Assist Resin Evacuation

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Substantial confusion and often deliberate misinformation has existed for over 25 years regarding resin evacuation methods with gas assisted injection molding. Resin evacuation in gas assisted molding has also casually been called “spillovers” or “overflows.”

The purpose of this information is to briefly explain the various methods, and to explain the advantages of AEGIS' PREP3 process whenever resin evacuation is needed.

Resin evacuation is a modification to gas assisted injection molding used for two primary reasons:

    1. To achieve a Class A product surface when molding with highly reinforced resins, and/or
    2. When an area in the part cavity fills early with resin, and a short-shot process cannot displace resin from this area of the mold cavity. Achieving a Class A surface in reinforced resin is 90% of the reason why resin evacuation is employed.

Resin Evacuation In the “Old Days” of Gas Assist Molding

The first method employed (late 1980's) was to have an open path from the mold cavity for the resin to “spill over” into upon gas injection. This occasionally worked, but offered nothing for molded part surface. The resin in the part cavity never achieved conventional molding pressure to “bury” the reinforcing material in the resin. The process was also inconsistent. This method of resin evacuation will never be recommended by AEGIS, and should never be employed.

Resin Evacuation In the Late 1990's

A process was developed by Alliance Gas Systems which used a method where a part cavity was filled with resin and packed by the molding machine, gas was then injected into the melt, (prepressuring the cavity) then, an evacuation path was opened for the resin to fill a plaque outside the parting line. This was a far superior process, but was soon improved as described below.

AEGIS' PREP3 Process

In 2000, a product came to our attention involving a tubular handle that was to be molded in 50% glass
reinforced PBT or PET, and required a high-gloss Class A surface. A short shot process would never
provided the needed appearance, and injecting gas into a prepacked cavity of 50% glass reinforced
polyester would not achieve the needed degree of resin evacuation one the evacuation path was opened.
The result was our developing what AEGIS calls “PREP3.” (3rd generation of resin evacuation).

The PREP3 process works as follows:

  1. The part cavity is filled completely and packed as though it were a conventional injection molded
    part. The evacuation path is closed at all times.
  2. The molding machine packing pressure is dropped
  3. A hydraulic cylinder retracts opening the flow path from the part cavity.
  4. Nitrogen gas is then injected at the lowest pressure that will adequately displace still-molten resin
    from the molded part

When using the PREP3 process as described above, ALL objectives are achieved, that is, achieving a Class A surface, and, based upon variation of the hold and pack (cooling) time, the remaining wall of the voided molding can be adjusted. Performed with our advice and implementation specifics, the process is fool proof. There is NO resin evacuation process that comes even close for repeatability and product consistency.

Productivity through gas assisted injection molding!

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Phone: 757-271-9927

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